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The ~ (tilde) operator triggers tilde expansion, a built-in shell parsing mechanism where Bash replaces a tilde-prefixed string with a specific absolute directory path. This expansion occurs strictly before variable expansion, command substitution, and pathname expansion, provided the tilde is unquoted and positioned at the beginning of a word or immediately following an unquoted = or : in a variable assignment. During parsing, Bash identifies an unquoted ~ and reads all subsequent characters up to the first unquoted slash / (or the end of the word). This extracted string is evaluated as the tilde-prefix. If the tilde-prefix matches a known expansion rule, the shell substitutes the prefix with the corresponding path. If the prefix is invalid or unresolvable, the string is left completely unchanged.

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Syntax and Expansion Variants

Current User Home Directory 
~
~/path
An un-suffixed tilde expands to the value of the $HOME shell variable. If $HOME is unset, Bash falls back to the home directory of the user executing the shell, resolved via the operating system’s password database (e.g., getpwuid()). Specific User Home Directory 
~username
~username/path
Expands to the absolute home directory associated with the specified username, resolved directly from the system’s password database (e.g., getpwnam()). Current Working Directory 
~+
~+/path
Expands to the value of the $PWD shell variable, representing the current working directory. Previous Working Directory 
~-
~-/path
Expands to the value of the $OLDPWD shell variable, representing the previous working directory. If $OLDPWD is unset, the expansion fails and the string remains literal. Directory Stack Expansion 
~+N
~-N
Expands to specific entries within the shell’s directory stack (managed via pushd and popd).
  • ~+N expands to the string that would be displayed by the dirs +N command (the Nth directory from the left of the stack, zero-indexed).
  • ~-N expands to the string displayed by dirs -N (the Nth directory from the right of the stack, zero-indexed).

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Assignment Context Parsing

Bash applies special parsing rules for tilde expansion during variable assignment. The expansion is triggered immediately following an unquoted = sign, and subsequently after any unquoted : characters within the assigned value. This is specifically designed to support PATH-like variable structures. 

# Tilde expansion occurs after '=' and ':'
VAR=~/dir1:~+/dir2

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Quoting and Suppression

Tilde expansion is strictly dependent on the ~ character remaining unquoted. If the tilde, or any character within the tilde-prefix, is enclosed in single quotes, double quotes, or escaped with a backslash, the expansion is suppressed and the tilde is treated as a literal character. 

# Expansion suppressed
\~
"~"
'~'
~"username"
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